Many different disciplines use motion analysis systems to capture movements and postures of the human body. To make realistic animations for movies and computer games, movements of the actor are captured and mapped onto a character. In sports, motion analysis techniques are used to analyze and improve performances. In the field of medicine and rehabilitation, recordings of human motion can be used, for example, to evaluate gait patterns.
Three dimensional (3D) motion capturing is generally performed using magnetic or camera-based systems. In camera-based systems, reflective or light-emitting markers attached to the body are observed by a number of cameras from which the 3D position can be reconstructed using triangulation of each camera 2D image. With magnetic trackers, magnetic field sensors measure the field as emitted by a source placed near the subject from which position and orientation of the magnetic field sensor with respect to the source can be calculated. The set-up of external emitters or cameras limits the working area where the subject can be captured and impedes many applications.
To capture the three dimensional human body movements and postures without the need for external emitters or camera's, miniature Orientation measurement units (OMU) can be placed on several body segments. These OMU can be arranged to measure their own motion with respect to an earth-fixed reference system or they can be arranged to measure relative orientation.
Orientation measurement units (OMU), notably inertial sensors, magnetic coils, goniometers, mechanical orientation sensing devices, or the like, can be used to record the movement of a subject i.e. an actor or a patient. The subject is described as an articulated body. By knowing the orientation of the segments as well as the relative distance between the joints, the complete body pose can be derived. An important requirement is that the pose orientation of the OMU with respect to the body segments must be known as well as the relative distances between joints.
In the known orientation measurement systems use is made of sequentially interconnected body portions connected by joints. To enable motion tracking, respective relative dimensions of the body portions constituting the said sequence must be known. The values of the dimensions are used for the distance between respective joints. Also, at least an orientation of the OMU with respect to the body portion must be known.
In order to enable motion tracking, the known motion tracking systems must be subjected to a step of calibration wherein both the orientation of the OMU's with respect to the body portions and at least a relative distance between the joints are determined. In an embodiment of the known motion tracking system such calibration step is performed by measuring the dimensions of the body portions and by collecting data from the OMU's when a person is standing in a standard, pre-known pose, like a T-pose. Different techniques exist for obtaining necessary data for calibration, notably the orientation of the OMU with respect to the body portion, of such known system:    a) Carefully securing the OMU in a known pose to the body portion of interest. Palpation of for example bony landmarks may be required to determine this pose;    b) Asking the subject whose movement is to be measured to stand in a known pose, for example upright with arms downwards, or a so-called T-pose.    c) Asking the subject to perform a certain movement that is assumed to correspond to a certain axis. For example, the arm axis is defined by a pronation or supination movement. Measured orientation, or other quantity such as angular velocity, is used to find the orientation of the OMU with respect to the body portion. Such a technique is for example described in Luinge et al J. Biomech. 2007; 40(1): 78-85. Epub 2006 Feb. 7.
Different techniques exist for obtaining necessary data for calibration, notably the relative distances between joints of the body portions, of such known system:
The distance between the joints can be obtained using regression equations to obtain body dimensions. One or a few body dimensions are measured using e.g. a measuring tape, or by using position measurements derived from a camera system. The measured dimensions can be used as input to known standard tables to estimate the joint positions.
It is a disadvantage of the known system and method for obtaining the orientation of the OMU with respect to the body portions as well as the relative distances between joints in that substantial inaccuracies occur due to errors in palpation or errors in acquiring and sustaining a static pose. Further, the known methods lacks subject specificity because only a few measures are taken and the joint position can not be measured directly, but must be derived using regression equations based on average human dimensions.
An inertial measurement unit (IMU) comprises gyroscopes, which measure angular velocities, accelerometers which measure accelerations including gravity and optionally magnetometers measuring the earth magnetic field. Using these signals, the orientation with respect to gravity and the Earth magnetic field and acceleration of the IMU housing can be obtained. An embodiment of a method an apparatus for motion tracking using IMU is known from U.S. Pat. No. 6,820,025.
The known methods for obtaining necessary data for calibration when using an IMU as OMU are particularly disadvantageous because as an additional requirement the subject has to stand in a known pose with respect to gravity as well as with respect to a local magnetic field. This latter condition places an extra burden on the subject.